Characterization And Antifouling Performance Of Double-skinned FO Membranes Modified With Graphene Oxide

As the hot spot in membranes separation technology,forward osmosis(FO)has received extensive attention recently.It has demonstrated technological advantages in the field of high salinity zero discharge,comprehensive utilization of seawater desalination and landfill leachate treatment.Nowadays,internal concentration polarization(ICP)is considered as one of the obstacles for the application of FO process.Double-skinned FO membranes has proved being efficient in mitigating the influence of ICP.In this study,polyether sulfone(PES)microfiltration membranes were used as the support layer coated by polyvinyl alcohol(PVA)by cross-linking polymerization and interfacial polymerization by polyamide(PA).In further,graphene oxide(GO)was used as fillers in PVA coating solution to improve the membrane performance.The result illustrated that the FO membranes modified with GO have more uniformitarian surface and more stable separation performance comparing with the control membrane samples.To a certain extent,the water flux surface characteristics and antifouling ability were improved.The parameters of the FO membranes were tested by standard test method.The water permeability,structural parameter,,reverse salt flux were used to evaluate the the membranes by standard test method and model foulants.Firstly,PVA cross-linking polymerization and interfacial polymerization were performed on the two sides of the PES support layer.The membrane structure,micro-morphology of active layer and antifouling ability of the double skinned membrane(S-PVA membrane)were vestigated.The results showed that the pores were visible for the S-PVA membranes after the PES support layers were coated.The roughness of the coated PVA layer was less than the PA polymerization layer.The hydrophilicity of the PVA layer greatly increased remarkably compared with the original PES membrane surface while the hydrophilicity of the layer interfacial polymerization decreased.During the membrane performance test of FO process with both orientations i.e.the active layer facing to feed solution side(AL-FS mode)and the active layer facing to the draw solution side(AL-DS mode),the membranes presented the maximum water flux of 17.01 LMH and 23.95 LMH,respectively when the concentration of coating PVA solution was both 0.4 wt%.And the values of the reverse salt flux was 6.3 g MH and 9.9 g MH,respectively.When the coating solution concentration was 0.8wt%,the membranes had the highest rejection rate of model foulants,i.e.94.8% of emulsified oil(W-oil),93.6% of bovine serum albumin(BSA),95.86% of sodium alginate(SA).Then,PVA coating was performed on both sides of the PES support layer and the PA interface polymerization was carried out in further to obtain a double skinned membrane(D-PVA membrane)in order to overcome the problem of the defects on the S-PVA membrane surface.The results showed that the PA layer formed uniformly on the PVA coated layer because the PVA cross-linking network covering on the MF pores provided a support for the PA matrix layer.Comparing to the S-PVA membranes,the active layer of PA illustrated more smoothly and uniformly.And the wettability test of PA layer showed that the PVA on the bottom layer effectively improved the hydrophilicity.The water flux of AL-FS model and AL-DS model also maintained at 17 LMH and 25 LMH,which was similar to S-PVA membranes.However,the fluctuation of reverse salt flux was smaller than that of S-PVA membranes.Finally,the double-skinned FO membranes was optimized through graphene oxide(GO)was mixed in the PVA coating solution.The performance and structural parameters of the PVA/GO membranes(D-PVA/GO membrane)were investigated.The results showed that the addition of GO effectively improved the PVA coating layer as PVA/GO cross-linked network was more compacted due to the GO nanosheets effectively covered the pores on the MF membrane surface,keeping PVA solution on the membrane surface.It also illustrated that the PVA solution mixed with increased the water flux.The maximum water flux of D-PVA/GO membranes increased 23.8% compared to the D-PVA membrane with various GO loading in PVA solution.The salt rejection rate was up to 98.5% while the rejection rate of model foulants,i.e.W-oil,BSA,SA,increased to 99.3%,98.9%,99.0% respectively.The structural parameters of the membranes were also smaller than the former two membranes,indicating that the membrane structure was more homogeneous with the addition of GO.The integrity and stability of the membrane structure were the most important factors in mitigating concentration polarization and improving anti-pollution properties.In conclusion,the membrane prepared by double-face modification can achieve higher performance in water and reverse salt flux.By comparasion,D-PVA membrane can effectively improve the stability of the membrane while maintained high permeability and antifouling ability.Through the addition of GO,D-PVA/GO membrane maintained good stability and high water permeability and salt rejection.Antifouling ability has enhanced significantly compared with the S-PVA and D-PVA membranes.Double-skinned FO membranes modified with graphene oxide are expected to make up the shortcomings of the commercial FO membranes.